The three-side trimming is usually carried out in a station by the block pressed between a cutting table and a pressing plate being trimmed first at the head and the foot and then on the front. The sequence may also be reversed. The knives cut by a pinching cut against a plastic strip. The knife penetrates minimally the cutting strip during the cutting. To reduce the cutting forces at the beginning of the cutting and the cutting forces in general, the movement of the knives generates an oblique swing cut, and the movement component on the side along the knife edge is approximately equal to the normal cutting movement. At the beginning of the cutting of the block, the knife edge is at first not parallel to the cutting table, as a result of which the cut begins at a comer of the block and the cutting force increases from a low value to the maximum.
Three-side trimmers, also called trimmers, are also used in which the head and foot cut and the front cut are performed in separate stations. This type is used mainly in the case of large numbers of cutting cycles but small product thickness. The division into two stations makes possible the high cut counts.
The cutting is usually performed according to the shears principle by a knife against a counterknife. The blocks are held only by pressing strips directly next to the cutting plane for fixation during cutting.
The three-side trimmers of the type mentioned first, which cut in one station, make possible cutting of high quality in case of a solid, heavy design of the components. However, the drawback is, besides the heavy design of the components, that the pressing plate and the cutting table must be accurately coordinated format parts. The cutting table is a replaceable part with very small format jumps. The pressing plate must be accurately adapted to the finished, cut format for each product. Moreover, roundings and bevels also frequently have to be arranged in the production of brochures to prevent creases in the back and of print marks on the pressing plate from being formed. This is especially uneconomical, particularly for short runs, where the set-up time plays an especially great role.
The usual, commercially available two-station three-side trimmers are not suitable for the production of products of high quality.
Aside from the limitation of the cutting thickness, the pressing strips produce marks, especially on the back in brochures. Because of the absence of full-area pressing, there are deviations in cut. These trimmers advantageously have no format parts.